1. Field of the Invention
The present invention relates to a reflection type liquid crystal display apparatus provided with reflecting electrodes.
2. Related Art of the Invention
A related art reflection type liquid crystal display apparatus is used in the TN or STN mode. The display apparatus is composed of a liquid crystal cell holding liquid crystal between two substrates on each of which transparent electrodes are formed; a pair of polarizing plates disposed so as to put this liquid crystal cell therebetween; and a reflecting plate disposed outside the polarizing plate of the lower substrate. In this configuration, however, a light passes through this pair of polarizing plates 4 times, so the display is dark. The transmittance of a light per polarizing plate is about 45% at the highest. At this time, the transmittance of a polarized light in parallel to the absorbing axis of the polarizing plate is almost 0% and the transmittance of vertically polarized light is almost 90%. In this configuration, therefore, the reflectance is found as follows; (0.9).sup.4 .times.50%=32.8%. The maximum reflectance will thus be about 33% even for a monochrome panel. Especially, in the case of a color liquid crystal display panel for which color filters are formed on one substrate in a monochrome panel configuration, the display will be darker than a monochrome panel, since the light is absorbed by the color filters. This is why it will become difficult to obtain a satisfactory brightness for a reflecting display.
In order to make the display brighter in such a configuration, there have been made some propositions so far. For example, only the upper polarizing plate is used on the liquid crystal cell and the liquid crystal cell is put between this polarizing plate and a reflecting plate (as disclosed in Unexamined Published Japanese Patent Application Nos. 07-146469 and 07-84252). In such a case, the light passes the polarizing plate only twice, so the reflectance per monochrome panel becomes (0.9).sup.2 .times.50%=40.5%. When compared with a two-polarizing-plate configuration, it can be expected that the reflectance is improved by about 23.5%.
Another proposition was to make the display brighter using a PCGH mode reflection type liquid crystal display panel (refer to H. Seki: 1996SID, P.614SID96DIGEST). In this case, no polarizing plate is used. The reflectance per monochrome panel is about 66% in this configuration and the display will become brighter than any of the above configurations.
When only one polarizing plate is used or when no polarizing plate is used as described above, metallic reflecting electrodes are formed on the lower substrate so that each of them is used as both a reflecting plate and an electrode, and a reflecting plate is disposed in the liquid crystal cell. This is to solve a parallax problem caused by the thickness of the substrate provided between the reflecting plate and the liquid crystal. An Al or Ag metallic material, which is low in wiring resistance and high in reflectance, is used for the metallic reflecting electrodes. The Ag material is expensive, but the Al material is used as a material of integrated circuits for general semiconductor devices (used to form elements on an Si wafer, for example). The Al material will thus be practical as a material of such metallic reflecting electrodes.
It is sure that a pure Al material is low in resistance and the most excellent as a metallic reflecting electrode material. When such an Al material is used for metallic reflecting electrodes, however, stress migration and electro-migration problems cannot be avoided. (The stress migration problem here means blistering (HIROKKU) of a thin film and wire breaking caused by a stress mainly in a heating process. The electro-migration means thin-film-like-wire breaking caused by electrical migration mainly at a power on operation (especially, a power-on operation under a high humidity and a high temperature).)
In order to solve the above problems, there have been developed various Al alloys usable as the above Al material. However, any of such Al alloys have not obtained resistance to stress migration and electro-migration enough to solve the above problems, since their resistance is increased and their reflectance is reduced.